Android 性能優化之佈局

Android 性能優化之佈局(一)

前言

Android佈局優化,是一個老生常談的問題,之前都知道要減小布局的層級,能夠UI優化,可是知其然不知其因此然,經過釋然小師弟的LayoutInflater源碼分析,我本身又去讀了一遍源碼,對於爲何這麼作能夠對性能有一個提高,在這裏從setContentView開始,整理一下思路,作一個記錄。java

源碼分析

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_layout)調用setContentView方法設置佈局xml
    init()
    }
    接下來咱們分析一下setContentView是怎麼設置xml
複製代碼

AppCompatActivity.javanode

/**
       * @return The {@link AppCompatDelegate} being used by this Activity.
       */

        @NonNull
        public AppCompatDelegate getDelegate() {
        if (mDelegate == null) {
        若是爲空,建立一個AppCompatDelete
        mDelegate = AppCompatDelegate.create(this, this);
        }
        return mDelegate;
        
      繼續向下追AppCompatDelegate是怎麼建立的
     /**
      * Create an {@link androidx.appcompat.app.AppCompatDelegate} to use with {@code activity}.
      *
      * @param callback An optional callback for AppCompat specific events
      */
        @NonNull
        public static AppCompatDelegate create(@NonNull Activity activity,
           @Nullable AppCompatCallback callback) {
           這裏return了一個Impl,繼續追過去看
        return new AppCompatDelegateImpl(activity, callback);
        }
        
      在AppCompatDelegateImpl中咱們會找到3個setContentView重載函數,這裏只貼出其中一個方法,有須要的同窗可自行查看源碼

        @Override
        public void setContentView(int resId) {
        ensureSubDecor();
        ViewGroup contentParent = mSubDecor.findViewById(android.R.id.content);
        contentParent.removeAllViews();
        //調用LayoutInflater把佈局放到contentParent裏面
        LayoutInflater.from(mContext).inflate(resId, contentParent);
        mAppCompatWindowCallback.getWrapped().onContentChanged();
       }
複製代碼

到此發現SetContentViwe是通LayoutInflater.from(mContext).inflate(resId, contentParent)將佈局文件放到了contentParent中。須要進一步分析LayoutInflater的源碼,看看是如何加載的xml,爲何佈局的層級會影響性能。android

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
    繼續查看inflate作了些什麼
    return inflate(resource, root, root != null);
    }
    
    public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
    final Resources res = getContext().getResources();
  
    if (DEBUG) {
        Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
                + Integer.toHexString(resource) + ")");
    }
    此處調用XmlResourceParser這是一個xml解析接口以獲取xml資源
    ,Android默認的xml解析器是XmlPullParser
    
    final XmlResourceParser parser = res.getLayout(resource);
    try {
    此處再次調用inflate的重載函數進行xml的解析
        return inflate(parser, root, attachToRoot);
    } finally {
        parser.close();
    }
}
//這裏貼出inflate重載函數的官方註釋
For performance reasons, view inflation relies heavily on pre-processing of
XML files that is done at build time. Therefore, it is not currently possible
to use LayoutInflater with an XmlPullParser over a plain XML file at runtime;
it only works with an XmlPullParser returned from a compiled resource

出於性能緣由,視圖的inflation是在構建的時候預編譯解析的。所以,當前沒法在運行時經過純XML文件在XmlPullParser中使用LayoutInflater。
此處個人理解就是佈局文件在編譯的時候,提取被解析成xml,下面看源碼中會發現,佈局文件解析是經過反射建立View對象

因爲代碼過長,貼出部分關鍵代碼
   public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
    synchronized (mConstructorArgs) {
        /**省略代碼**/
        //定義返回值,初始化值爲傳入的root
        View result = root;

        try {
            // Look for the root node.
            int type;
            while ((type = parser.next()) != XmlPullParser.START_TAG &&
                    type != XmlPullParser.END_DOCUMENT) {
                // Empty
            }

            if (type != XmlPullParser.START_TAG) {
                throw new InflateException(parser.getPositionDescription()
                        + ": No start tag found!");
            }//對START_TAG,END_DOCUMENT,事件進行解析處理

            final String name = parser.getName();//獲取當前的事件標籤


            if (TAG_MERGE.equals(name)) {
            
                if (root == null || !attachToRoot) {
                    throw new InflateException("<merge /> can be used only with a valid "
                            + "ViewGroup root and attachToRoot=true");
                }
                //若是使用了merge標籤,那麼就在此進行遞歸處理。使用merge標籤需注意,必需要有父佈局而且要依賴父佈局加載,否則會報異常
                rInflate(parser, root, inflaterContext, attrs, false);//此處圈起來
            } else {
                 //若是沒有使用merge,那麼建立一個臨時的根視圖temp
                // Temp is the root view that was found in the xml
                final View temp = createViewFromTag(root, name, inflaterContext, attrs);

                ViewGroup.LayoutParams params = null;

                if (root != null) {
                    if (DEBUG) {
                        System.out.println("Creating params from root: " +
                                root);
                    }
                    // Create layout params that match root, if supplied
                    params = root.generateLayoutParams(attrs);
                    if (!attachToRoot) {
                        // Set the layout params for temp if we are not
                        // attaching. (If we are, we use addView, below)
                        temp.setLayoutParams(params);//臨時的params
                    }
                }
                  /**省略代碼**/
                  //遞歸子佈局
                   rInflateChildren(parser, temp, attrs, true);
                // Decide whether to return the root that was passed in or the
                // top view found in xml.
                if (root == null || !attachToRoot) {
                    result = temp;//將臨時根視圖賦值給開始定義的返回值result
                }
            }
           /**省略代碼**/
        return result;
    }
}

final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
        boolean finishInflate) throws XmlPullParserException, IOException {
    rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}

到這裏會發現不論哪一種方式,最終都會調用rInflate()函數加載xml而後在這個函數中又會調用一個很重要的函數createViewFromTag(),接下來分析一下這個函數

View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
        boolean ignoreThemeAttr) {
    if (name.equals("view")) {
        name = attrs.getAttributeValue(null, "class");
    }

    // Apply a theme wrapper, if allowed and one is specified.
    if (!ignoreThemeAttr) {
        final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);
        final int themeResId = ta.getResourceId(0, 0);
        if (themeResId != 0) {
            context = new ContextThemeWrapper(context, themeResId);
        }
        ta.recycle();
    }
       if (name.equals(TAG_1995)) {
      // Let's party like it's 1995!
        return new BlinkLayout(context, attrs);
    }

        try {
        View view;
        //調用Factory來建立View
        if (mFactory2 != null) {
            view = mFactory2.onCreateView(parent, name, context, attrs);
        } else if (mFactory != null) {
            view = mFactory.onCreateView(name, context, attrs);
        } else {
            view = null;
        }

        if (view == null && mPrivateFactory != null) {
            view = mPrivateFactory.onCreateView(parent, name, context, attrs);
        }

        if (view == null) {
            final Object lastContext = mConstructorArgs[0];
            mConstructorArgs[0] = context;
            try {
                if (-1 == name.indexOf('.')) {//含有"."tag的控件  好比一些自定義view
                    view = onCreateView(parent, name, attrs);
                } else {
                    view = createView(name, null, attrs);
                }
            } finally {
                mConstructorArgs[0] = lastContext;
            }
        }

        return view;
    } 
		/**省略代碼**/
}
這裏首先會用Factory.onCreateView獲取一個View對象,若是獲取不到,最終會調用createView()函數建立View對象

      public final View createView(String name, String prefix, AttributeSet attrs)
        throws ClassNotFoundException, InflateException {
      Constructor<? extends View> constructor = sConstructorMap.get(name);
    if (constructor != null && !verifyClassLoader(constructor)) {
        constructor = null;
        sConstructorMap.remove(name);
    }
    Class<? extends View> clazz = null;

    try {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);

        if (constructor == null) {
            // Class not found in the cache, see if it's real, and try to add it
            clazz = mContext.getClassLoader().loadClass(
                    prefix != null ? (prefix + name) : name).asSubclass(View.class);

            if (mFilter != null && clazz != null) {
                boolean allowed = mFilter.onLoadClass(clazz);
                if (!allowed) {
                    failNotAllowed(name, prefix, attrs);
                }
            }
            constructor = clazz.getConstructor(mConstructorSignature);
            constructor.setAccessible(true);
            sConstructorMap.put(name, constructor);
        } else {
            // If we have a filter, apply it to cached constructor
            if (mFilter != null) {
                // Have we seen this name before?
                Boolean allowedState = mFilterMap.get(name);
                if (allowedState == null) {
                    // New class -- remember whether it is allowed
                    clazz = mContext.getClassLoader().loadClass(
                            prefix != null ? (prefix + name) : name).asSubclass(View.class);

                    boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
                    mFilterMap.put(name, allowed);
                    if (!allowed) {
                        failNotAllowed(name, prefix, attrs);
                    }
                } else if (allowedState.equals(Boolean.FALSE)) {
                    failNotAllowed(name, prefix, attrs);
                }
            }
        }

        Object lastContext = mConstructorArgs[0];
        if (mConstructorArgs[0] == null) {
            // Fill in the context if not already within inflation.
            mConstructorArgs[0] = mContext;
        }
        Object[] args = mConstructorArgs;
        args[1] = attrs;
       //根據獲取的構造器實例化一個View的對象
        final View view = constructor.newInstance(args);
     
        if (view instanceof ViewStub) {
            // Use the same context when inflating ViewStub later.
            final ViewStub viewStub = (ViewStub) view;
            viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
        }
        mConstructorArgs[0] = lastContext;
        return view;
      }
複製代碼

總結

1.從第一步在onCreate中調用setContentView設置xml開始,一步一步查看源碼,能夠看到xml經過Layoutinflater解析實例化成一個View,是須要XmlPullParser解析xml,Java反射來操做建立View,這是一個很耗時的過程。若是佈局層級過於複雜,那麼遞歸調用所消耗的數據也就越長,執行反射的次數也會增長,就會帶來性能上的問題。
2.使用merge標籤能夠減小布局的嵌套,減小了佈局層級,那麼建立view的時候可讓遞歸調用的時間縮短,反射的次數減小,從而達到一個性能的優化。 3.使用約束佈局ConstraintLayout,能夠有效減小布局時候的嵌套問題性能優化

固然,UI性能的優化不止於佈局層級方面,View的過分繪製的優化,使用ViewStub進行懶加載,使用AsyncLayoutInflater異步加載佈局等。Android10中新增了一個函數tryInflatePrecompiled,能夠有效的解決佈局方面優化的問題,這個函數我就在此不作贅述了,能夠移步下方連接查看。app

寫在最後

感謝 「Android10源碼分析」爲何複雜佈局會產生卡頓?-- LayoutInflater詳解 釋然小師弟異步

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